(1) Field of the Invention
The present invention relates to a method of linear optoacoustic communication and optimizing the communication between an in-air platform and an undersea vehicle by selecting various parameters such as power, modulation frequency and a laser wavelength to increase the communication range to an undersea receiver.
(2) Description of the Prior Art
In 1881, Alexander Graham Bell discovered that acoustic energy could be generated when high intensity light impinged on various media, such as water. This acoustic energy generation is typically called the “optoacoustic effect”.
In the optoacoustic effect, high intensity light is exponentially attenuated by the impinged medium resulting in local temperature fluctuations that give rise to volume expansion and contraction. These expansions and contractions generate a propagating and measurable pressure wave.
The energy transfer of the optoacoustic effect can be divided into a linear regime and a non-linear regime on the basis of energy density and irradiance imparted to the medium. The linear regime occurs when the acoustic pressure is proportional to the laser power and the physical state of the medium does not change. The linear optoacoustic regime generally uses intensity modulated laser sources of relatively low energy density. The effect of the optical absorption of the laser light in the medium is to produce an array-like arrangement of thermoacoustic sources that generate a modulated pressure wave at the laser amplitude modulation frequency.
In contrast to the linear regime, the non-linear regime occurs when the acoustic pressure depends non-linearly on the laser power and the physical state of the medium does change.
The benefits of the optoacoustic effect (optoacoustic transduction) are: in-water transducer hardware is not required; transmit frequency and directivity patterns can be controlled by a proper laser/modulation selection; and a moving optoacoustic source does not generate flow noise at any speed.
The linear optoacoustic conversion process as well as sound attenuation due to spreading loss and absorption are well documented. However, the equations describing both sound attenuation scenarios have not been analyzed to determine the laser parameters for optimizing communications range based on parameters of the medium, bandwidth, and a received signal to noise ratio (SNR). Previous optoacoustic work also did not consider the underwater communication application or the optimization of a generated acoustic source in order to increase in-water range.
To efficiently evaluate linear and non-linear techniques, a need exists for improved acoustic communication performance for both normal and oblique laser beam incidence in terms of range and data rates based on laser systems that are either practical or commercially available or whose parameters have been optimized through simulation for angular coverage as well as for in-air range and in-water range.